The present invention relates to a system for entrenching elongated structures, such as pipelines, on the bottom of the sea.
With the discovery of heavy concentrations of off-shore mineral deposits, numerous procedures and equipment have been developed for laying pipelines for transporting the minerals either to shore or to an off-shore collection location. The pipelines are connected between the locations and laid along the bottom of the sea.
For the protection of both the pipelines from the surrounding environment and the environment from the pipelines, techniques have been developed for burying the pipelines within trenches along the sea bottom. While various government regulations require that the pipelines be entrenched, the cost of the operation due to various drawbacks with previous trenching equipment has been a significant element in the initial start-up cost. Some of the considerations necessitating trenching of the pipeline are the concerns of protecting the pipeline from damage from storms, mudslides and from entanglement with fishing nets or other large objects being dragged along the sea bottom such as anchors and subsequent damage to the sea due to the leakage in the pipes. The necessity of finding improved techniques and equipment for entrenching the pipelines has been increasingly emphasized over the past several years.
Ideally, a rectangular trench, one having side walls forming a 90 degree angle with the sea bottom, should be formed. However, even in hard clays, it is difficult if not entirely impossible to form such rectangularly shaped trenches. As the trench is cut, a crumbling effect occurs along the side walls, i.e., the side walls cave inwardly thereby causing the walls to slope outwardly. Hence, the realistic goal is to minimize the slope of the side walls, i.e., maximize the angle of repose (the angle of repose is that angle that a side wall forms with a horizontal plane lying along the bottom of the trench). The depth of the trench is generally a minimum of 3 feet plus the diameter of the pipe in order that once the pipe is entrenched, the top of the pipeline lies 3 feet below the plane of the natural sea bottom.
Several different types of trenching devices are presently utilized for entrenching pipelines. Such devices include sea sleds with fluid jet systems, mechanical diggers and mechanically operated plows. The water jet sea sleds are sleds that straddle the pipelines and are dragged along as fluid jets break down the cohesion of the soil. The broken down soil is then removed from the trench by means of eductors. Such sea sleds are described in the above noted Chang, et al. patents. The mechanical diggers are either towed from the surface or self-propelled along the sea bottom so as to move along the pipeline. The mechanical diggers include mechanical cutting devices that actually cut through the soil and carry it off to a location outside of the trench. The third type of system, the mechanical plows also can be either towed from the surface or self-propelled along the sea bottom. The systems merely include shaped members which move along the path of the pipeline so as to plow out a trench in the same manner that a trench would be plowed on the surface.
With all three types of systems, there are several problems that arise during the trenching operation. In order to form a sufficiently sized trench for entrenching the pipeline, it is often necessary to make multiple passes with the trenching equipment over the pipeline. The necessity of multiple passes increases both the costs of the trenching operation and the risk of damage to the pipe. When the equipment is utilized in softer soils, such as fine sand, the trenches often are formed with an extremely small angle of repose. The smaller the angle of repose the greater the chance that the pipe will not be buried with soil due to natural redeposition processes. During the trenching operation, the spoil which is removed from the trench frequently slips back into the trench before the pipe reaches the trench bottom, which increases the number of passes that must be made in order to properly entrench the pipeline. In addition, when trenching in soft soils, such as sand, there is a tendency for the entire trench forming apparatus (e.g., the sled) to be undermined and to ride in the trench. This problem is especially predominant where multiple passes must be made over the pipeline. One further problem that is encountered is the difficulty of properly guiding the trench former over the pipeline due to faulty readings from the sensing mechanism utilized in guiding the sled over the pipeline.
With the fluid jet sea sleds, the broken-up soil is carried off by a spoil removal mechanism. In such arrangements, such as described in the previously noted patents to Chang, et al., the sea sled is provided with jet nozzles directed at sea bottom and jet type eductors. The jet eductors remove the cuttings or slurry, formed by the jet nozzles, from the trench. Such sea sleds can be used in any depth of water including depths exceeding 200 feet. The eductor system as described in the patents to Chang et al. include a pair of suction conduits mounted on a frame work that straddles the pipeline to be entrenched. Each suction conduit has an inlet at its lower end for receiving the slurry formed by the operation of the jet nozzles and a discharge conduit at its other end for discharging the slurry into the ambient water on the side of the trench. The discharge conduit is located so as to discharge the slurry in a direction substantially parallel to the sea bottom on the side of the trench.
During the operation of the fluid jet sea sled, there are several difficulties that are incurred that often necessitate that multiple passes be made over the pipeline in order to form a sufficiently sized trench. During the excavation operation, as the sled moves along the pipeline, a significant portion of the trench may be refilled. It previously had been believed that the refilling of the trench ocurred primarily due to a crumbling or caving effect along the side walls of the trench. In the development of the present invention, it now has been realized that the refilling may be due in large part to the discharge spoil reentering the trench before the pipe reaches the trench bottom. Such refilling of the trench necessitates the makings of multiple passes over the pipeline in order to ensure proper entrenching.
In order to ensure that the soil was adequately broken up the jets expelled from each nozzle have typically been of a relatively high pressure, on the order of 2,000 psi. The utilization of such high pressure fluids, however, significantly increases the cost of providing pumping equipment capable of generating large volumes of such high water pressure for expulsion through the jet nozzles, with this cost being on the order of millions of dollars. In addition, if high pressure fluids are utilized, special tubing must be employed for carrying the high pressure fluids from the vessel down to the jet nozzles while with lower pressure fluids less expensive tubing could be utilized. In addition, when utilizing high pressure fluids in certain types of soft soil, e.g., fine sands, the high pressure jets sometimes can stir up too much of the sand thereby leaving it temporarily suspended in the water and decreasing the effectiveness of the eductor mechanism for removing the soil. In order to compensate for this last problem, it has been suggested that the spacing between the jet tube and eductor be increased by moving the eductor further back on the sled, so that the sand settles before attempting to remove it by the eductors.
Other problems that have been incurred during the operation of the sea sleds have arisen due to the movement of the sled along the sea bottom. If the sled is traveling either too high or too low on the pipeline, then the depth of the trench will vary thereby leading to a cratering or spanning effect of the pipeline as it passes across valleys and other hills on the trench sea bottom. One additional reason for such valleys and hills in the trench is the effect of any heaving or pitching movement of the vessel as it tows the sled along the sea bottom. In addition, if the sled is riding too low on the pipeline or if too wide a trench has been formed, then the sled will sink into the trench thereby rendering the trenching operation increasingly difficult and also causing the bottom of the trench to take a form of a wave.
The mechanical diggers and mechanical plows suffer from some of the same drawbacks and difficulties encountered with the fluid jet sea sleds. During the operation of the mechanical diggers and the mechanical plows, in order to form a properly sized trench, it is necessary for such equipment to make multiple passes over the pipeline. In addition, such equipment must be extremely massive since it is primarily dependent upon the force imparted by the mechanical equipment upon the soil. Hence if the equipment impacts against the pipeline, it can cause severe damage to the pipeline. Furthermore, such equipment is expensive to construct and often must be specifically designed for each type of trenching operation thereby further increasing the cost of the trenching process.